Field of the Invention
The present invention relates generally to a telemetry system for use in medical devices of various types and for various purposes implantable within the human body, and more particularly to a telemetry system which will transmit data at a relatively high rate in both directions while retaining a high degree of accuracy.
With the ever-shrinking size of electronic circuitry, the implantation into the human body of electronic medical devices has become more and more common. Although the most commonly known of such devices is the cardiac pacemaker, there are a variety of devices which are implanted into the human body, including devices for the stimulation or tenting, or both, of the brain, nerves, the spinal cord, muscles, bones, glands, or other body organs or tissues.
It will be appreciated by those skilled in the art that such implantable devices are becoming increasingly complex, and that more functions can be crammed into a relatively smaller electronic chip or circuit. Implantable medical devices have used bidirectional telemetry for a number of years. Information transmitted between the implanted device and an external transceiver may include, for example, device identification information, biological data, current operational parameters of the device, technical information regarding proper operation of the device, battery charge condition, revised operational parameters (programming information) for the device, and verification of information transmitted between the implanted device and the external transceiver.
With an ever-increasing amount of data being processed and available within the implantable device, there has been a corresponding increase in the need to transmit more data from the implanted device to the external transceiver for analysis, reprogramming of the implantable device, or other purposes. The need for more data to be transmitted in both directions has increased tremendously the amount of time required both to interrogate the implanted device and to reprogram the implanted device. An upper limit on the amount of data flowing between the implanted device and the external transceiver has therefore become directly proportional to the amount of time which may reasonably be taken to interrogate and reprogram the implanted device.
Accordingly, it is desirable to achieve a higher rate of data transfer between the implanted device and the external transceiver to eliminate this artificial impediment and to maximize the communication between the implanted device and the external transceiver. Of course, an increase in the rate of data exchange may not be obtained at the expense of accuracy in a medical device, particularly if the device is a life-sustaining device such as a pacemaker. Absolute accuracy is required, and it is apparent that an increase in the rate of data transfer will be nullified by an increase in the amount of time spent to verify data to ensure the degree of accuracy required.
The recent development of LSI circuits incorporating low current analog-to-digital converters has made the use of such converters in implantable devices possible. There are, of course, limitations surrounding the design of new implantable systems or portions thereof, the most limiting of which is the consumption of energy by the system. Implanted systems are customarily powered by a long-lasting non-replaceable internal battery, and the current consumption of a telemetry sub-system thus becomes perhaps the most important design factor to be considered.
Previously known devices have established various methods of communicating non-invasively through the skin. For example, U.S. Pat. No. 4,223,679, to Schulman et al., which patent is assigned to the assignee of the present invention, shows an implanted device which uses little or no current to transmit information by relying on reflected impedance of an internal L-C circuit. Internal modulation circuits in the implanted device transmit digital or analog data by modulating the reflected impedance, and the external transceiver utilizes an oscillator having varying frequency and amplitude outputs determined by a coupled RF magnetic field carrier to an L-C circuit in the external transceiver. This system works well, but, unfortunately, has speed limitations making it unsuitable for transmitting the amount of information contemplated herein.
Another type of device uses an active type transmitter, with the transmitted energy being taken from the implantable device battery. This type of device, which is illustrated in U.S. Pat. No. 4,281,664, to Duggan, has a data rate which is limited to approximately 100 BPS (bits per second). Another device which utilizes an active type transmitter is shown in U.S. Pat. No. 4,453,162, to Money et al. The Money et al. device can not achieve more than 0.25 to 0.05 bps/carrier cycle, a rate too low for the applications contemplated herein.
Those skilled in the art will also appreciate that it is not practical to increase the carrier frequency above approximately eight kHz, since the metal enclosure of the implanted device will experience eddy currents great enough to attenuate the signal significantly. In addition, increasing the carrier frequency makes electromagnetic interference from video display terminals, which are generally approximately 16 kHz, a significant problem. Consequently, techniques requiring a number of carrier cycles for each signal bit will be capable of achieving a speed only two to four kBPS.
It will therefore be appreciated that there exists a substantial need for a telemetry system which is capable of accurately transmitting and receiving data at a rate enabling the substantial amounts of data used by current implantable systems to be conveniently sent in a relatively short period of time. The amount of power required by the implanted portion of the system must be minimal, so as not to adversely affect battery life. Finally, the system should be compact so as to not add significantly to the space required by the implanted device.